Screening is used to separate solids according to particle size and or to separate solids from fluids. The solids to be screened may be dry or wet and may often be screened from a flowable solids and liquids mixture (slurry). The process is used in many industries including: mineral and metallurgical processing, quarrying, pharmaceuticals, food and the drilling of oil, water and gas wells. The design of screening equipment varies widely but will generally be of one of two types, either static or moving.
Static screens generally include coarse screens and sieve bends. These are normally mounted at an angle such that solids on the screen roll over it by gravity and in so doing either pass through the screen or roll off it. Static screens are typically used to screen down to 5 mm. Sieve bends may be used to screen finer sizes.
Moving screens are generally described according to the motion of the screen. Types will typically include: revolving rotary screens, shaking screens, gyratory screens, linear screens and high frequency vibratory screens. Moving screen arrangements normally have two elements, the screen panel and the screening machine.
Screen panels will generally be mounted in the screening machine in such a manner that they may be removed and replaced either when worn or damaged or when a change in separation size is required. Screen panels may be constructed of widely differing materials, including but not limited to, woven wire mesh, wedge wire, moulded plastics, synthetic woven fabrics and drilled plates of either plastic or metal. Screen panels are made with different hole sizes to provide separation at different sizes.
The function of the screen panel is:                To retain solids above screen aperture size on the panel.        To transmit the motion generated within the screening machine to the solids and liquid, such that the fluid passes through the screen and the solids retained on the screen are transported on the screen to a point of discharge from the screen.        To allow fluid and solids under screen aperture size to pass through the screen.        To ideally offer resistance to blinding and plugging of the screen apertures from solids that are of similar size to the screen aperture size.        
The screening machine design will vary widely according to the movement that it is required to impart to the screen panel, the number of screen panels, the method of feeding the panels, the process application, working environment and process capacity required. The screening machine motion will normally be arranged to impart energy to the screen panel such that:                Solids under screen aperture size are moved in such a manner that encourages them to pass through the screen. These solids are termed ‘undersize’        Solids that are larger than the screen aperture and as such cannot pass through the screen are retained by the screen and transported off the screen. These solids are generally termed ‘oversize’. Any fluid discharged from the screen with the oversize solids is generally termed ‘screen overflow’.        Fluids carrying solids are encouraged to pass through the screen. Fluid passing through the screen is generally termed ‘screen underflow’.        
Moving screens are used for the screening of either dry or wet solids and or the screening of solids from fluids. Dry screening will typically be used for separation of dry solids down to 1 mm diameter. For sizes lower than 1 mm, wet screening will normally be used. This method eliminates dust. Wet screening will normally be the screening of solids from a flowable slurry, being a mixture of solids and a fluid (liquid).
Where a slurry is screened to remove the majority of the fluid from the solids, without any specific need to size the solids, the function of the screen is generally termed ‘dewatering’. This term is applied to the function of the machine and will apply to slurries that are made with water or any other liquid as the fluid.
Where slurry is screened to achieve a specific size split the function of the screen is termed ‘classification’.
In addition to screening equipment making use of screen panels as described above, other types of solids/liquids separators can be used, for example centrifuges such as decanting centrifuges, to separate a solids/liquids mixture.
Whilst screening machines, especially vibratory screening machines such as the so called ‘shale shakers’ of the oil well drilling industry are used with success in methods of solids/liquids separation, especially classification, there is a need to improve throughput and effectiveness. This is especially the case where available space is severely limited, for example on offshore oil rigs, and the option of increasing equipment size or the numbers of machines employed may not be available.
During the drilling of an oil well, fluid known as mud is circulated, under pressure, inside the drilling assembly to the drill bit. One of the functions of the drilling mud is to carry the rock cuttings generated during the drilling process at the drill bit, out of the borehole.
The constitution of drilling mud varies according to the mud type. Generally the mud will contain a fluid phase and a solids phase. The solids phase may include a weighting agent such as Barite that is added to the fluid to control the density of the mud. Other weighting agents can be employed. Generally weighting agents are made of materials that are of high specific gravity, typically within the range of 3.2 to 4.4 SG. The weighting agent will normally be an inert material that will have minimum impact on the viscosity and fluid properties of the drilling fluid when added in various concentrations. The size of the weighting agent particles will normally be below 74 microns with the majority of the particles being under 40 microns diameter. As the weighting agent is added to the drilling mud to control the density of the drilling mud during use, it is generally desirable that the weighting agent is not removed from the mud system but retained within it. Other desirable solids can be incorporated into the mud system such as ‘Bridging’ and ‘Lost Circulation Material’. These solids will generally be of within a desirable size range such that they perform the function for which they are designed.
When the drilling mud arrives at the drilling rig the solids fraction of the mud will contain desirable solids and drilled solids. The drilled solids are generally undesirable solids comprised predominantly of rock but can contain metal fragments. The drilled solids are undesirable as these are generally rock cuttings that if allowed to accumulate at increased concentrations result in undesirable effects on the fluid properties of the mud. As the concentrations of drilled solids in a mud increases the fluid properties are affected until the mud becomes unusable and requires replacement or the addition of new mud to dilute the concentration of drilled solids such that the desired fluid properties are restored. The removal and control of the concentrations of drilled solids is generally regarded as a most important activity in contributing to the successful, safe and economic drilling of an oil well, within the planned time and cost.
The process of removal of drilled solids must remove drilled solids while leaving desirable solids such as weighting material within the fluid. Drilled solids are conventionally removed from the mud using first shale shakers to screen the fluid. Rock cuttings above screen size are removed during screening and the fluid passes into storage tanks for subsequent mechanical and chemical processing, where this is desirable, and ultimate recirculation to the oil well. After screening at the shale shaker additional solids separation techniques can be applied to remove any drilled solids that have passed through the shale shaker, being smaller than the screen size fitted to the shale shaker.
These techniques conventionally include the use of hydrocyclones of various sizes and centrifuges. A large diameter hydrocyclone is conventionally termed a Desander and smaller diameter hydrocyclones is conventionally termed a Desilter. The terms Sand and Silt used in the context above are geological terms referring to the size of the particle concerned. Sand in generally above 74 microns diameter and silt can range down to a few microns in diameter. Centrifuges can be of varying types and configuration, decanting centrifuges are typically employed to separate fine drilled solids. A combination of decanting centrifuges can be used to recover weighting agents and remove drilled solids.
Solids control equipment typically removes solids within the following size ranges:
Conventional Shale shakersSolids above 74 microns.High efficiency Shale ShakersSolids above 40 microns.DesandersSolids between 1000 and 74 microns.DesiltersSolids between 74 and 10 microns.Decanting centrifugesSolids between 200 and 5 microns.
When choosing the type of equipment to be employed to remove and control the concentration of drilled solid in the mud the following are generally accepted desirable criteria:
The process should be as simple as possible.
Drilled solids should be removed at the earliest possible opportunity when they are at their largest size.
Pumping, recirculation to the oil well and aggressive handling that results in the fracture of the drilled solid into smaller particles must be avoided, as small solids are significantly more difficult to remove from the mud than large solids.
Drilled solids should not be allowed to be recirculated to the oil well as during recirculation they will be broken down and become increasingly difficult to remove.
The minimum equipment necessary to achieve the function should be employed.
Equipment should be easy to operate for the operators thereof.
The installed system should ideally be of low weight, size and power consumption.
The system should offer high efficiency of separation.
The system should be reliable.
The efficiency of drilled solids removal should be easily measured.
Desanders, desilters and centrifuges suffer from the following undesirable features:                A feed tank containing feed mud is required this is generally large and heavy,        A feed pump is required resulting in high power requirements, maintenance, weight and space.        During pumping of the drilled solid it is normally fractured and reduced in size making it significantly more difficult to remove from the mud.        Basis of separation is by the mass of the cutting not size. Desirable solids such as weighting material are of high specific gravity. Drilled solids are generally of lower specific gravity material within the range of 2.8-2.2 sg. The mass of a weighting agent particle can be similar to the mass of a much larger drilled solid, resulting in the hydrocyclone separating both desirable weighting material and undesirable drilled solids of similar mass. It will be noted that this problem does not occur with screening as the screen separates by size.        Separation efficiency is variable as fluid properties vary.        Separation efficiency is difficult to measure.        Decanting centrifuges capital cost and maintenance cost are high.        
Shale shakers are conventionally employed in preference to other equipment due to the following characteristics                No feed tank required.        Equipment is simple for the operator to understand and easy to operate and maintain.        Installed space and weight and typically low.        Power consumption is low.        Basis of separation is size.        Separation efficiency is easily determined being directly relative to the mesh size fitted.        Separation efficiency is not variable with fluid properties provided the fluid passes through the mesh size fitted.        
The drilling mud returning to the drilling rig from a well normally contains a low concentration of drilled solids within a large volume of fluid. The drilled solids removal system is thus required to process a large volume of fluid to remove a small volume of drilled solids. Consequently the size of a drilled solids removal system has historically been directly relative to the volume of fluid to be processed and NOT the volume of solids to be removed.
The oil industry has previous employed hydrocyclone and screen (e.g. in shale shakers) combinations to concentrate the volume of solids into a smaller volume of fluid. One such typical apparatus is called a mud cleaner. Mud cleaners typically employ hydrocyclone assemblies mounted above a shale shaker or shakers. Mud is pumped to the hydrocyclone, where the mud is split into two streams, the hydrocyclone overflow, comprising cleaned fluid and the hydrocyclone underflow containing fluid and drilled solids that is passed to the shaker for removal of oversize solids. Analysis of the performance of the mud cleaner has demonstrated that low solids removal efficiencies resulted due to the following:                Drilled solids were fractured into smaller particles during pumping to the hydrocyclone resulting in them becoming increasingly difficult to separate.        Separation efficiency was highly variable, dependant on mud fluid properties.        The hydrocyclone was easily overloaded with solids. When overloaded drilled solids were returned to the mud system in the cone overflow thus bypassing the separation system.        Monitoring the separation efficiency of the hydrocyclone was difficult and complex.        
The analysis also demonstrated that the efficiency of separation achieved by the fine screen element of the mud cleaner was consistently high, determinable and easy to monitor in the field. Historically this analysis led the industry away from hydrocyclone/screen combinations and towards the development of higher capacity shale shakers such as the AX1 Shale Shaker manufactured by Axiom Process Limited.
One or more shale shakers are used depending upon the volume of fluid being pumped and the separation efficiency required. Generally as finer screens are fitted to the shale shaker the process capacity of the shaker decreases while the efficiency of separation of solids increases. Typically screening will take place using screens, generally made of woven wire mesh, of between 10 and 400 mesh. These screens will contain between 10 and 400 wires per inch respectively and aperture hole size will vary according to the weave pattern and diameter of the wire used in the weave.
To achieve the required process capacity and separation efficiency a drilling rig shale shaker installation will typically contain between one and eight shale shakers although some installations can employ more machines. Machines will be employed to work in parallel where the fluid from the oil well is split into multiple streams and processed by an equal number of machines. Installations of shale shakers can thus be appreciable in size.
Alternatively an installation can contain multiple machines working sequentially (in series), each separating at a progressively finer size. Alternatively an installation can contain a combination of machines working in parallel and series.
The need to design a vibratory screening machine to provide the required fluid throughput while transporting solids to the point of discharge from the screen has resulted in conventional machines being of a larger size or in greater numbers than is ideal where space and weight are restricted by either physical or economic factors.
An object of the current invention is to provide methods and apparatus that can significantly increase the processing capacity of a screening system allowing the size of the system to be significantly reduced, relative to a conventional approach, for a given process capacity.
The invention herein relates to a method and equipment for improving the volumetric capacity of wet screening equipment. Typically the equipment will be used for performing a classification function and typically the solids particle size range will be of the order of between 10 mm and 10 microns. However the methods and apparatus may be used for other solids/liquids separations, with particle sizes out with that range.
Improvements to the versatility and throughput of vibratory screening machines are described in WO/2004/110589 (PCT/GB2004/002544—Axiom Process Limited) wherein vibratory screening machines including a stack of screen assemblies mounted in a vibrating basket for solids/liquid separation are described. The improved machines include a flow distributor arrangement that can allow parallel processing through two screens mounted in a stack thereby increasing throughput. The flow distributor can allow both parallel and series processing and thereby increases the scope of possible operations of a given size of machine. Typically such machines are employed for separating out solids from a solids and liquid feed (used drilling mud) to allow recycling of a cleaned fluid stream, disposal of unwanted solids and in some cases recovery of solids of a selected size range for reuse.
The full contents of WO/2004/110589 are incorporated herein by reference.
Despite the improvements described above there is still a need to further improve apparatus and methods for screening solids and liquids mixtures, especially but not exclusively in drilling operations, for example in offshore environments where space is at a premium and the drive to drill under ever more varied and demanding conditions benefits by the provision of space efficient, versatile and robust equipment.